It is generally accepted, that increasing oil prices and national countries' legislations that require the reduction of automotive carbon dioxide emissions force tire and rubber producers to contribute to produce “fuel efficient,” and thus fuel or gas saving tires. One general approach to obtain fuel efficient tires is to produce tire formulations that have reduced hysteresis loss. A major source of hysteresis in vulcanized elastomeric polymers is believed to be attributed to free polymer chain ends, that is, the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in any efficient elastically recoverable process, and as a result, any energy transmitted to this section of the polymer is lost. This dissipated energy leads to a pronounced hysteresis under dynamic deformation. Another source of hysteresis in vulcanized elastomeric polymers is believed to be attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its Tan δ, at 60° C., value (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small Tan δ values, at 60° C., are preferred as having lower hysteresis loss. In the final tire product, this translates to a lower rolling resistance and better fuel economy.
One generally accepted approach to reducing hysteresis loss is to reduce the number of free chain ends of elastomeric polymers. Various techniques are described in the open literature including the use of “coupling agents,” such as tin tetrachloride, which may functionalize the polymer chain end and react with components of an elastomeric composition, such as for example with a filler or with unsaturated portions of a polymer. Examples of such techniques, along with other documents of interest, are described in the following patents: U.S. Pat. Nos. 3,281,383; 3,244,664 and 3,692,874 (for example, tetrachlorosilane); U.S. Pat. No. 3,978,103; U.S. Pat. Nos. 4,048,206; 4,474,908; U.S. Pat. No. 6,777,569 (blocked mercaptosilanes) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon such as 1,3,5-tri(bromo methyl)benzene); U.S. Pat. No. 4,616,069 (tin compound and organic amino or amine compound); and U.S. 2005/0124740.
The application of “coupling agents,” as reactant to living polymers, more often than not, leads to the formation of polymer blends comprising one fraction of linear or uncoupled polymers and one or more fractions comprising more than two polymer arms at the coupling point. For example, silicon tetrahalide can be mentioned as one typical representative of silicone halide based coupling agents. The application of silicon tetrahalide in a less than one to one halide to living polymer chain ratio usually leads to the formation of polymer blend fractions comprising branched three arm and/or four arm polymers of relatively high molecular weight, and to a polymer blend fraction of non-branched polymers of comparably low molecular weight. The function of the branched polymer blend fraction is to reduce the elastomeric polymer hysteresis. The function of the relative low molecular weight non-branched polymer fraction is to optimize polymer processing properties. End-functionalization of the non-branched polymer blend fraction can be performed in another process step, further decreasing the polymer hysteresis attributed to polymer chain end to polymer interactions, or to polymer chain end to filler interaction. Both polymer to polymer, and/or polymer to filler interactions, as observed in the case of the linear chain-end modified polymer fraction, do not occur, or do not occur to the same extent, as in case of the branched polymer blend fraction. Therefore, it is desirable to incorporate one or more group(s) into the coupling agent, which are reactive with the filler particle surface, for example with groups located on a silica surface or a carbon black surface. Generally, it is desirable to incorporate one or more group(s), which are reactive with the filler particle surface, into all polymer molecules present in an elastomeric polymer blend or present in an elastomeric polymer composition.
“Synthesis of end-functionalized polymer by means of living anionic polymerization” Journal of Macromolecular Chemistry and Physics 197 (1996), 3135-3148, describes the synthesis of polystyrene-containing and polyisoprene-containing living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps, obtained by reacting the living polymer with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as protecting group for the —OH and —SH functions in the termination reactions, because the corresponding silyl ethers and thioethers are found to be both, stable and compatible with anionic living polymers.
International Publication No. WO2007/047943 describes the use of a silane-sulfide modifier represented by the formula (RO)x(R)ySi—R′—S—SiR3 wherein x is the number one, two or three, y is the number zero, one or two, the sum of x and y is three, R is alkyl and R′ is aryl, alkylaryl or alkyl, to produce a chain end modified elastomeric polymer used as component in a vulcanized elastomeric polymer composition or in a tire tread.
More specifically, according to WO2007/047943, a silane-sulfide compound is reacted with anionically-initiated, living polymers to produce chain end modified polymers, which are subsequently blended with fillers, vulcanizing agents, accelerators or oil extenders, to produce a vulcanized elastomeric polymer composition having low hysteresis loss. To further control polymer molecular weight and polymer properties, a coupling agent (or linking agent) can be used according to WO 2007/047943, as optional component, in the process of the preparation of elastomeric polymers. The modifier is than added before, after, or during, the addition of a coupling agent, and preferably, a modification reaction is completed after the addition of the coupling agent. In some embodiments, more than a third of the polymer chain ends are reacted with a coupling agent prior to addition of the modifier.
There is a need for modification methods and resulting modified polymers that can be used to further reduce hysteresis loss. These needs have been met by the following invention.